As monitoring the pressure of tires is essential to the safety of motor vehicles, pressure-monitoring systems, such as TPMS systems, were created to perform this function and installed on motor vehicles.
These primarily comprise sensors that measure the pressure of the tires and transfer it to the central unit. This central unit collects and analyzes the tire pressure data via wireless links. The result of these analyses is generally displayed on the dashboard of the vehicle. Such a system is therefore able to inform the driver of the tire pressure in real time, or just produce a visual or audio announcement if pressure anomalies are detected by this system. It is also possible for the driver to receive this information on a mobile telephone.
This may be performed by way of frequency-shift keyed signals. Frequency-shift keying is known under the acronym FSK. In the following text, frequency-shift keying will be used to denote this type of digital frequency modulation, which is performed between predetermined frequencies.
One example of a wheel unit communicating with a central unit through frequency-shift keying in order to measure a pressure or the temperature inside a tire is given for example by document US 2015/377741, incorporated herein by reference.
As mentioned above, a method for assisting in the inflation of tires of a vehicle is performed when the vehicle is stationary, this being given the name in immobilization mode. In these cases of the vehicle being stationary, a major problem of a risk of poor transmission between the tires and the central unit arises.
Specifically, when the wireless links between the pressure sensors of the wheel units and the central unit encounter obstacles, the level of reception of the signals at the central unit decreases to the point that these signals may become inaudible. The positions of the wheels that generate these reception problems are called black spots.
If the stoppage of the vehicle corresponds, for one or more wheels, to black spot conditions, the transmission remains impacted throughout the entire duration of the stoppage, as the position of the wheel does not vary. These black spots may be caused for example by the position of the transmitter antenna of a wheel unit, by reflection on the ground of part or of all of the transmitted signal, or by the relative position of the wheel unit with respect to the central unit.
Experience has shown that, in a stationary position, the probability of the wheel unit of at least one wheel being in a black spot position is significant and non-negligible. When a wheel unit is stationary at a black spot, the budget of its link to the central unit is greatly degraded, up to around −90 dBm. In this case, the signal received by the central unit is no longer able to be analyzed due to the excessively low signal-to-noise ratio.
No prior art document has taken into account this problem of stopping on a black spot for frequency-shift keyed signals. Common practice is to search, for frequency-shift keying, for a predetermined frequency band that is as narrow as possible. This was supposed to optimize frequency-shift keying communication and guarantee it in all conditions, which is clearly not the case for a black spot.
Document FR 3 002 492, incorporated herein by reference, describes a self-adaptive method for assisting in the inflation of tires of a vehicle, consisting in continuously measuring at least the pressure of each tire by way of a wheel unit, and in transmitting signals on the state of the tires via a wireless link between each wheel unit and a central unit at least two different bit rates, called high bit rate and low bit rate. The central unit then continuously analyzes these signals and then transmits information in relation to the state of the tires to an information module. A continuous measurement of the shift/immobility state of the vehicle is also provided to the central unit and to the wheel unit.
Although decreasing the bit rate proves to be effective, given that replacing a high-bit rate transmission with a low-bit rate transmission allows an improved signal-to-noise ratio and enables a reception sensitivity gain, thus allowing the signals to be received, this document does not give any indication with regard to modifying parameters of signal frequency-shift keying that could solve the problem of defective transmission due to the presence of a black spot for at least one of the wheels of a stationary motor vehicle.